The present invention concerns a tuning system for a satellite receiver, especially one capable of receiving and processing television signals transmitted in digital form.
Satellite television receiving systems usually comprise an xe2x80x9coutdoor unitxe2x80x9d including a dish-like receiving antenna and a xe2x80x9cblockxe2x80x9d converter, and an xe2x80x9cindoor unitxe2x80x9d including a tuner and a signal processing section. The block converter converts the entire range (xe2x80x9cblockxe2x80x9d) of relatively high frequency RF signals transmitted by a satellite to a more manageable, lower range of frequencies.
In a conventional satellite television transmission system, television information is transmitted in analog form and the RF signals transmitted by the satellite are in the C (e.g., 3.7 to 4.2 GHz) and Ku (e.g., 11.7 to 14.2 GHz) bands. The RF signal received from the satellite by the antenna of the receiving system are converted by the block converter to the L band (e.g., 900 to 2000 MHz). An RF filter section of the tuner of the indoor unit selects the one of the RF signals received from the block converter corresponding to the selected channel, and a mixer/local oscillator section of the tuner converts the selected RF signal to a lower, intermediate frequency (IF) range for filtering and demodulation.
In newer satellite television systems, such as the DirecTv(trademark) operated by the Hughes Corporation of California, television information is transmitted in digital form. The RF signals are transmitted by the satellite in the Ku band, and are converted by the block converter to the L band. The frequency range of the RF signals transmitted by the satellite is somewhat smaller (e.g., between 12.2 and 12.7 GHz) than that for the analog satellite television system, and the frequency range of RF signals produced by the block converter is accordingly somewhat smaller (e.g., between 950 and 1450 MHz).
In a digital satellite television broadcast system, the television information is digitized, compressed and organized into a series or stream of data packets corresponding to respective video, audio, and data portions of the television information. The digital data is modulated on to a RF carrier signal in what is known as QPSK (Quaternary Phase Shift Keying) modulation and the RF signal is transmitted to a satellite in earth orbit, from which it is retransmitted back to the earth.
In QPSK modulation, the phases of two quadrature phase signals, I and Q, are controlled in response to the bits of respective digital data streams. For example, the phase is set to 0 degrees (xc2x0) in response to a low logic level (xe2x80x9c0xe2x80x9d), and the phase is set to 180xc2x0 in response to a high logic level (xe2x80x9c1xe2x80x9d). The phase shift modulated I and Q signals are combined and the result transmitted as a QPSK modulated RF carrier signal. Accordingly, each symbol of the modulated QPSK carrier indicates one of four logic states, i.e., 00, 01, 10 and 11
The conversion stage of the block converter of the outdoor unit usually includes an RF local oscillator which is not stabilized against variations caused by temperature and aging. The result is that the frequency of the local oscillator signal of the block converter changes, causing a corresponding change or offset of the frequencies of the carrier signals of the RF signals received by the tuner of the indoor unit. As a consequence, the frequency of the near base-band signal produced by the tuner also changes or is offset from its nominal value. If the frequency of the near base-band signal deviates from its nominal value, the digital signals modulated on the near base-band signal cannot be properly demodulated and the information they represent cannot be properly reconstructed.
When the deviation is within a pull-in range of the digital carrier recovery loop (CTL) of the QPSK demodulator, the undesirable offset may properly be corrected by the phase lock loop operation of the CTL. If the frequency of the near base-band signal changes too far from its nominal value (i.e., outside the pull-in range of the CTL), the tuning system changes the frequency of the tuner local oscillator (LO) in order to bring the offset signal within the pull-in range of the CTL.
It is noted, however, that during the process for searching for the offset signal, the CTL may erroneously be locked at a point known as a xe2x80x9cfalse lock pointxe2x80x9d where the phase error is sufficiently small such that the system appears to be locked but no real offset signal exists. As a result, the tuning system will spend a significant amount of time determining whether or not the CTL has properly acquired the real offset signal by checking other indicators. This undesirably slows down the entire operation of the tuning system.
In order to solve the problems addressed above, the present invention concerns an offset frequency correction system, wherein searching for an offset signal starts from an end point of a band of frequencies in which the offset signal may be found.